The "Door" in Kingdom Hearts?

I love Kingdom Hearts and know almost the whole story except for one thing.. In Sora's awakening in KH1, the voice (who I here is actually King Mickey?) says "You are the one who will open the door." But what is the door? Is he talking about the Door to Light at the end of KH2? And what is that door in the secret place of Destiny Islands? Is that the one that Riku opened on the night of the storm? Thanks.